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Working with Nix Shell

Nix shells give you an ephemeral shell environment with packages installed for you to use. This is useful to try out things and with mkShell you can describe and recreate such an environment whenever you need it.

The nix-shell command is an old style command with a dash in between. During the transition to the next version of Nix, there's no way around these commands and it's good to at least know that they exist.

Shells with -p

Using nix-shell with -p (short for --packages) can get you quickly set up with a shell with packages from Nixpkgs installed.

$ nix-shell -p hello wget

# nix sub shell with hello installed
[nix-shell:~/nix-shorts]$ which hello
/nix/store/some-hash-hello-2.10/bin/hello

[nix-shell:~/nix-shorts]$ hello
Hello, world!

[nix-shell:~/nix-shorts]$ exit
exit

# back to your top level shell
$

Expressions as arguments

But instead of only top level attributes, you can also put entire expressions in the -p argument. This is more of an advanced use, but it's included here so that you've seen it once.

For example, with the easy-purescript-nix project:

$ nix-shell \
     -p 'let ep = import ./default.nix {}; in [ ep.purs ep.spago ]' \
    --run 'which purs; which spago; purs --version; spago version'
/nix/store/some-hash-purescript/bin/purs
/nix/store/some-hash-spago/bin/spago
0.13.3
0.9.0.0

As mentioned before, we can use Nix shells to get specific versions of packages.

$ nix-shell -p '(import ./default.nix {}).purs' --run 'which purs; purs --version'
/nix/store/some-hash-purescript/bin/purs
0.13.3
$ nix-shell -p '(import ./default.nix {}).purs-0_13_0' --run 'which purs; purs --version'
/nix/store/some-hash-purescript/bin/purs
0.13.0

Nixpkgs mkShell

To work with Nix shells in some organized manner, you probably will want to use a source-controlled file defining the shell. To make this concise and explicit, you can use mkShell from Nixpkgs.

Instead of having the expression passed on the command line, nix-shell will load the expression from a file. If no file is passed explicitly, nix-shell will look for either a file named shell.nix or a file named default.nix in that order.

Here's a way get the same shell we just had:

# src/shell/shell.nix
{ pkgs ? import <nixpkgs> {} }:

pkgs.mkShell {
  buildInputs = [ pkgs.hello pkgs.wget ];
}

And we can see this used like this:

$ nix-shell --run 'which hello; hello' # implicitly does `nix-shell shell.nix --run ...`
/nix/store/some-hash-hello-2.10/bin/hello
Hello, world!

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